The invention relates generally to joining processes and, more particularly, to methods for joining powder metallurgy components during sintering.
Powder metallurgy (xe2x80x9cP/Mxe2x80x9d) fabrication methods are becoming increasingly more widespread as an alternative to other metalworking technologies. In particular, metal injection molding (xe2x80x9cMIMxe2x80x9d) is a P/M fabrication method that allows net-shape or near-net shape production of components close to full density. Similar to injection molding of thermoplastic polymers, MIM can produce components with complex shapes that would otherwise require extensive machining.
The method typically involves forming a mixture of MIM powders with a binder and injecting the mixture into a mold. Once the green part is ejected from the mold, the binder is removed by a solvent and/or a thermal process. The resulting brown part is then consolidated by sintering.
While MIM can advantageously be used to make components having complex shapes, the process has been generally limited to components having sizes between about 1 and 200 grams. MIM components are usually not joined to each other to form assemblies because conventional joining methods often result in poor bond strength. Sinter bonding, for example, as disclosed in U.S. Pat. No. 5,554,338 is a method for joining P/M components by diffusion bonding. In this method, two compacts in the green or brown state are joined during the sintering process by forming metallurgical diffusion bonds between the P/M components. Diffusion bonds, however, form only at local contact points. Because the brown or green parts have rough bonding surfaces, diffusion bonding at only local contact points may result in poor bond strength.
MIM components can also be joined by conventional sinter brazing methods. Bonds resulting from sinter brazing, however, are generally between 5,000 to 10,000 microns in thickness because of excessive infiltration of filler material into the pores of the P/M components to be joined. Since the filler metal has a different composition compared to the joined P/M components, excessive infiltration not only affects the mechanical properties of the assembly, but results in poor bond strength.
Thus, there is a need to overcome these and other problems of the prior art and to provide methods for forming assemblies by bonding P/M components. The present invention, as illustrated in the following description, is directed to solving one or more of the problems set forth above.
In accordance with an embodiment of the present invention, a joining method is disclosed. The method includes providing a first and a second powder metallurgy compact each having a bonding surface and a bonding agent including a binder and fine particles. The bonding agent is placed between the bonding surfaces of the first and second powder metallurgy compacts. The first and second powder metallurgy compacts are then consolidated during a sintering cycle in which the first and second powder metallurgy compacts are joined by at least solid state diffusion of the fine particles.
In accordance with another embodiment of the present invention, another joining method is disclosed. The method includes providing a first and a second powder metallurgy compact, wherein the powder metallurgy compacts have similar composition and are formed by metal injection molding. Each powder metallurgy compact has a bonding surface. A bonding agent including a water-based binder and fine particles is placed between the bonding surfaces of the first and second powder metallurgy compacts. The first and second powder metallurgy compacts are consolidated during a sintering cycle in which the first and second powder metallurgy compacts are joined by forming a bond having an essentially similar composition to the first and second powder metallurgy compacts.
In accordance with another embodiment of the present invention, an assembly is disclosed. The assembly include a first powder metallurgy component, at least a second powder metallurgy component, and a bonded joint between the first powder metallurgy component and the at least a second powder metallurgy component formed by solid state diffusion and effectuated by a binding agent including fine particles.